The present invention relates to injecting CO.sub.2 into a reservoir containing swelling clay. More particularly, the invention provides a method for increasing the oil recovery obtainable by injecting an oil mobilizing and oil displacing proportion of CO.sub.2 into an oil containing reservoir having a combination of permeability and swelling clay content capable of significantly impeding the injection of heated or unheated aqueous fluid or unheated CO.sub.2.
It is commonly known that CO.sub.2 can be injected in various types of oil reservoirs in order to increase the amount of oil recovery from either cyclic or continuous oil displacement processes by becoming dissolved in the oil and increasing its mobility and/or displacing the oil into a production location within the reservoir. In addition, CO.sub.2 has been injected into reservoirs at various temperatures for various reasons, for example, as described in the following patents: U.S. Pat. No. 3,442,332 relates to using a combination of producing CO.sub.2 while producing ammonia, and using the CO.sub.2 to recover oil by injecting it at the lowest temperature at which it provides a producible oil viscosity at a suitable injection pressure. U.S. Pat. No. 4,042,029 describes producing oil from an extensively fractured reservoir by injecting CO.sub.2, heated if desired, into a gaseous zone overlying a liquid zone within the reservoir and producing oil from the liquid zone. U.S. Pat. No. 4,325,432 describes a process of injecting internal engine combustion gas treated with mangenese or manganese dioxide, at temperatures greater than 400.degree. F., into an oil or oil shale reservoir. U.S. Pat. No. 4,429,744 describes a process of injecting CO.sub.2 in steam, or in slugs alternated with steam, while using a specified schedule of production pressure recycling in a fluid drive oil production process.
But, where an oil reservoir has a combination of permeability and swelling clay content capable of significantly impeding the injection of steam or other hot or cold aqueous fluid or unheated CO.sub.2 in order to increase the mobility of the oil and its displacement toward a production location; as far as the Applicant is aware, the problem of how to effect an economical recovery of the oil has heretofore remained unsolved.